(a) Technical Field
The present invention relates to a humidification apparatus for a fuel cell system. More particularly, the present invention relates to a humidification apparatus for a fuel cell system, which can control the amount of dry air to be humidified by a membrane humidifier based on operating conditions of the fuel cell system.
(b) Background Art
One of the most attractive fuel cells for a vehicle is a polymer electrolyte membrane fuel cell (PEMFC), also called a proton exchange membrane fuel cell, which includes: a membrane electrode assembly (MEA) including a polymer electrolyte membrane (PEM) for transporting hydrogen ions and an electrode catalyst layer, in which an electrochemical reaction takes place, disposed on both sides of the PEM; a gas diffusion layer (GDL) for uniformly diffusing reactant gases and transmitting generated electricity; a gasket and a sealing member for maintaining airtightness of the reactant gases and coolant and providing an appropriate bonding pressure; and a bipolar plate for transferring the reactant gases and coolant.
In a fuel cell having the above-described configuration, hydrogen as a fuel and oxygen (air) as an oxidant are supplied to an anode and a cathode through flow fields of the bipolar plate, respectively. The hydrogen is supplied to the anode and the oxygen (air) is supplied to the cathode.
The hydrogen supplied to the anode is dissociated into hydrogen ions (protons, H+) and electrons (e−) by a catalyst of the electrode catalyst layer provided on both sides of the electrolyte membrane. At this time, only the hydrogen ions are selectively transmitted to the cathode through the electrolyte membrane, which is a cation exchange membrane and, at the same time, the electrons are transmitted to the anode through the GDL and the bipolar plate, which are conductors.
At the cathode, the hydrogen ions supplied through the electrolyte membrane and the electrons transmitted through the bipolar plate meet the oxygen in the air supplied to the cathode by an air supplier and cause a reaction that produces water. Due to the movement of hydrogen ions occurring at this time, the flow of electrons through an external conducting wire occurs, and thus a current is suitably generated.
Furthermore, the PEMFC requires water for its operation and, to this end, the air (or oxygen) supplied to the cathode of the fuel cell is humidified by a humidifier. Although there are various humidification methods such as bubbler, steam injection, adsorption, etc., a membrane humidifier having a relatively small volume is widely employed in the fuel cell vehicle due to spatial limitations. The membrane humidifier also does not require any power.
FIG. 1 is a schematic diagram showing a typical humidification apparatus for a fuel cell system, which humidifies air using a membrane humidifier. As shown in the figure, external dry air is forcibly passed through a membrane humidifier 10 by an air blower 1 and, at this time, supersaturated humid air containing water discharged from an outlet of a fuel cell stack 20 passes through the membrane humidifier 10 such that the dry air is humidified by water exchange between the supersaturated humid air and the dry air and the humidified air is supplied to the fuel cell stack 20.
A typical membrane humidifier is a gas-to-gas membrane humidifier that employs hollow fiber membranes. In this type of membrane humidifier, the hollow fiber membranes having a high contact surface area can be highly integrated, and thus it is possible to provide sufficient humidification to the fuel cell stack with a small capacity. Moreover, since the water and heat contained in the gas discharged from the fuel cell stack are collected and reused by the membrane humidifier, it is possible to save water and energy consumed for the humidification of the fuel cell stack.
One of the factors that has a significant effect on the performance during the operation of the fuel cell is to supply a sufficient amount of water to the electrolyte membrane and ionomers in the catalyst layer, which are the key components of the fuel cell, to maintain moisture content, thus maximizing the ionic conductivity of the electrolyte membrane and the ionomers. Here, the membrane humidifier serves to supply moisture and heat contained in high-temperature gas discharged from the fuel cell stack to dry reaction gas at room temperature supplied to the fuel cell stack via the membrane surface, thus achieving the humidification of the fuel cell stack and maintaining the temperature of the fuel cell stack.
Next, the structure of the membrane humidifier will be described in detail.
FIG. 2 is a perspective view showing a typical membrane humidifier, and FIG. 3 is an exploded perspective view of the membrane humidifier. As shown in FIGS. 2 and 3, a membrane humidifier 10 includes a membrane module 11 with a structure, in which hollow fiber membranes 11c are fixed in a case 11a, and a first housing 13 and a second housing 14 which are assembled on both ends of the case 11a of the membrane module 11 and include inlets 13a and 13b and an outlet 14a, respectively.
Here, the hollow fiber membranes 11c are arranged in the form of a bundle in the case 11a of the membrane module 11, and end ends of the hollow fiber membrane bundle 11c are fixed at both inner ends of the case 11a by potting portions 11b, and thus the position of the hollow fiber membrane bundle 11c is fixed in the case 11a. 
However, the above-described conventional membrane humidifier for the fuel cell has the following problems.
Although sufficient humidification is required in a low current region of the fuel cell, a large amount of water is also produced in high power and high current regions, which increases the mass transfer resistance in the cathode. This may cause flooding and the large amount of water blocks the air supply, which results in air starvation in the cathode. As a result, the deterioration of the fuel cell catalyst and high current regions, but the difference in the amount of humidification between the high current region and the low current region is insignificant in the conventional membrane humidifier and a high humidity of 80% RH is provided even in the high current region, which is similar to that in the low current region. Moreover, it is impossible to control the amount of humidification by the membrane humidifier itself based on operating loads of the fuel cell. Furthermore, the flow rate of air increases in the high current region, which increases the pressure drop and the load of the air blower, which is problematic.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.